The need for precise control of A/F ratio in motor vehicles has let to the development of controllers in which all or a portion of the engine air flow and exhaust system dynamics are mathematically modeled to estimate the sensed A/F ratio, and to adaptively adjust both the model and the base fuel control based on deviations of the estimated A/F ratio from the sensed A/F ratio. See, for example, SAE Paper N. 950846, by Fekete, Guden and Powell, entitled Model-Based Air-Fuel Ratio Control of a Lean Multi-Cylinder Engine. However, such controls tend to be complex, and when updated in time with the engine firing events, present excessive computational throughput requirements at higher engine speeds. Accordingly, such control strategies tend to be cost-prohibitive for most applications.